Mirror for planet-hunting space telescope arrives

The Kepler planet-hunting space telescope has taken another step towards completion with the delivery for testing of the spacecraft's main mirror. The telescope is designed to find Earth-size planets around other stars and is scheduled to launch in June 2008.

The telescope will watch for slight dips in brightness that occur when planets pass in front of their parent stars. It will stare at the same patch of sky for four years, simultaneously monitoring the brightness of 100,000 stars.

The telescope will trail the Earth in a Sun-centred orbit in which it will gradually drift farther from Earth. By the end of its mission, it is expected to be up to 75 million kilometres away.

Kepler's main mirror is 1.4 metres across and was made by L-3 Communications Brashear, an optics company based in Pittsburgh, Pennsylvania, US. It is the largest mirror ever built for a space mission that will travel beyond Earth orbit. It has been delivered to Ball Aerospace in Boulder, Colorado, US, which is assembling the spacecraft.

"Arrival of the technologically advanced Kepler mirror is an important milestone," says David L Taylor, president and CEO of Ball Aerospace. It will now undergo environmental tests before being integrated into the telescope's structure.

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Kepler may be the first telescope to detect an Earth-size exo-planet at the right distance from its star to support life (Illustration: NASA)